Patent Document 1, indicated below, discloses a vehicle wherein an oil filter for a vehicle-mounted internal combustion engine is installed in a filter case that is integrally formed with a case cover of a crankcase. The vehicle disclosed in Patent Document 1 is a rough-terrain four-wheel drive (4WD) vehicle. Some vehicle-mounted internal combustion engines for saddle-type vehicles or the like are smaller in size and challenged by the task of securing an oil filter space.